1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of a thin film transistor (hereafter referred to as TFT), and to a method of manufacturing thereof. In particular, the present invention relates to the structure of each pixel in a pixel region forming a display portion, and to the structure of a driver circuit for transmitting a signal to the pixels. For example, the present invention relates to an electro-optical device, typically a liquid crystal display panel, and to electronic equipment loaded with this type of electro-optical device as a part.
Note that, throughout this specification, semiconductor device denotes a general device which can function by utilizing semiconductor characteristics and that the category of semiconductor devices includes electro-optical devices, semiconductor circuits, and electronic equipment.
2. Description of Related Art
A liquid crystal display device is known as an image display device. Due to the fact that a higher definition image can be obtained in comparison with a passive type liquid crystal display device, active matrix type liquid crystal display devices have come into widespread use. The structure in the active matrix type liquid crystal display device has the orientation of a liquid crystal controlled by application of a voltage to pixels arranged in a matrix shape, and image information displayed on a screen.
The use of this type of active matrix type liquid crystal display device is propagating widely, beginning with portable information terminals, such as a notebook type personal computer (note PC), a mobile computer, and a portable telephone, and continuing to various types of electronic equipment, such as a liquid crystal television. Compared to a CRT, it is possible to make this type of display device lighter weight and thinner, and depending upon its use, there is a demand for giving the screen a large surface area and increasing the density of pixels.
Techniques of forming portions of a TFT, such as a channel forming region, by using an amorphous semiconductor film, typically amorphous silicon, have superior productivity. The amorphous semiconductor film has the characteristic of being able to be formed on a relatively low cost, large surface area substrate, such as barium borosilicate glass and aluminum borosilicate glass. However, the largest value of the electric field effect mobility that can be obtained in a TFT in which the channel forming region is formed from the amorphous silicon film is only on the order of 1 cm2/Vsec. The TFT can therefore be used as a switching TFT (a pixel TFT) formed in the pixel region, but cannot be given the desired operation when forming a driver circuit. Consequently, the driver circuit for controlling the voltage applied to the pixels in accordance with a signal uses an IC chip (a driving IC) manufactured on a single crystal silicon substrate, mounted in the periphery of the pixel region by a TAB (tape automated bonding) method or a COG (chip on glass) method.
The TAB method is a method of packaging in which a wiring is formed on a flexible insulating substrate from a material such as copper foil, an IC chip is installed directly on top, and one edge of the flexible substrate is connected to an input terminal of the display device. On the other hand, the COG method is a method of connecting in which the IC chip is directly joined in accordance with a wiring pattern formed on the substrate of the display device.
Further, the techniques of mounting the driver circuit on the display device substrate, as disclosed in Japanese Patent Application Laid-open No. Hei 7-0148880 and Japanese Patent Application Laid-open No. Hei 11-160734, in which a driver circuit is formed from a TFT manufactured by a non-single crystal semiconductor material on a substrate such as glass or quartz, and partitioned into strips (such substrates having a driver circuit cut into a strip shape are hereafter referred to as stick drivers), have been disclosed as other methods of mounting the driver circuit.
Whichever method is used, it is preferable to make the region in which the driver circuit is mounted as small as possible on the substrate forming the pixel region, and various designs have been ingeniously made for the method of driver circuit mounting, including the wiring layout.
CRTs have been used most as televisions and personal computer monitors. However, as the CRTs are replaced by the liquid crystal display devices from the viewpoint of saving space and of lower power consumption, while making liquid crystal display devices larger in size and higher in definition is promoted, there is also a demand for lowering their production cost.
An active matrix type display device uses a photolithography technique for manufacturing a pixel TFT, and at least five photomasks are used. A photomask is used in order to form a photoresist pattern on a substrate, which becomes a mask for an etching step in the photolithography technique. Using one photomask leads to processes such as applying resist, pre-baking, exposure, development, and post-baking, and in the steps before and after, there are processes such as film formation and etching, and in addition, processes such as resist peeling, supplemental cleaning, and drying. Consequently, the work related to manufacturing becomes complicated, which is a problem.
Reduction of the number of process steps is considered as an effective means in order to increase productivity and to increase yield. However, there is also a limit in the reduction of manufacturing cost if the number of photomasks is not reduced.
Further, static electricity is generated by causes such as friction during manufacturing steps because the substrate is an insulator. If static electricity is generated, then short circuits develop an intersection portion of wirings formed on the substrate, and deterioration or breakage of the TFT due to static electricity leads to display faults or deterioration of image quality in electro-optical devices. In particular, static electricity develops during rubbing in the liquid crystal process performed in the manufacturing steps, and this becomes a problem.
In addition, if the number of pixels increases, then the number of IC chips installed will also inevitably become large. With an RGB full color display XGA panel the number of terminals on the source line side of the pixel region alone becomes approximately 3000, and 4800 are necessary with UXGA. The size of the IC chip is limited by the wafer size in the manufacturing process, and the practical size limit of the longer side is on the order of 20 mm. Even with an output terminal pitch of 50 μm, one IC chip can only cover 400 connection terminals. Approximately 8 IC chips are required on the source line side only in the above XGA panel, and on the order of 12 are necessary for the UXGA panel.
A method of manufacturing a long size IC chip has also been considered, but the number of strip shape IC chips which can be cut out from a circular shape silicon wafer is naturally lowered, and therefore the method is not practical. In addition, the silicon wafer itself has a fragile nature, and if a rather long IC chip is manufactured, then the probability of failures increases. Furthermore, the mounting of the IC chips requires precise placement of the same and reduction in contact resistance of the terminal portion. If the number of IC chips joined to one panel increases, then the likelihood of defects developing increases, which leads to a fear of reducing the yield. In addition, the temperature coefficient of the silicon which becomes the substrate of the IC chip differs from the temperature coefficient of the glass substrate on which the pixel region is formed, and therefore problems such as warping develop after the two substrates are joined. This becomes a cause of a lowering in the reliability of the element due to the developed mechanical stress, as well as of direct defects such as an increase in the contact resistance.
On the other hand, it is possible to form the driver circuit with a length equal to that of the pixel region by using the stick driver, and mounting of the driver circuit can be performed by forming one stick driver. However, if the amount of surface area of the circuit portion increases, the number of stick drivers which become defective due to a single point defect increases, and therefore the number which can be cut out of one substrate is reduced, inviting a reduction in the process yield.
From the standpoint of productivity, a method of forming a plurality of stick drivers from TFTs manufactured from a crystalline semiconductor film on a large surface area glass substrate or quartz substrate is considered superior. However, the driving frequency differs between the scanning line side and the source line side, and further, the value of the driving voltage applied also differs. Specifically, the TFTs in the stick driver of the scanning line side must withstand on the order of 30 V, while the driving frequency is equal to or less than 100 KHz, and therefore no high speed characteristics are required. A voltage resistance on the order of 12 V is sufficient for the TFTs in the stick driver of the source line side, but high speed operation on the order of a driving frequency of 65 MHz at 3 V is required. Thus it is necessary to make the structure of the stick driver and the TFTs within the drivers different due to the different specifications required.
Based on this background, the realization of a reduction in steps for manufacturing a pixel TFT of a liquid crystal display device, reducing manufacturing costs and increasing yield, is a first object of the present invention. Further, a method of manufacturing a driver circuit, formed from TFTs which satisfy the characteristics required by each circuit and which are formed at the same time on a large surface area substrate such as a glass substrate; providing a display device which mounts that type of driver circuit; and providing a technique of improving reliability and productivity, are second objects of the present invention.